The conventional image-forming process of silver halide photography includes imagewise exposure of a photographic silver halide recording material to actinic radiation (such as actinic light), and the eventual manifestation of a useable image by wet photochemical processing of that exposed material. A fundamental step of photochemical processing is the treatment of the material with one or more developing agents to reduce silver halide to silver metal. With black-and-white photographic materials, the metallic silver usually comprises the image. With color photographic materials, the useful image consists of one or more images in organic dyes produced from an oxidized developing agent formed wherever silver halide is reduced to metallic silver.
To obtain useful color images, it is usually necessary to remove all of the silver from the photographic element after color development. This is sometimes known as "desilvering". Removal of silver is generally accomplished by oxidizing the metallic silver, and then dissolving it and undeveloped silver halide with a "solvent" or fixing agent in what is known as a fixing step. Oxidation is achieved with an oxidizing agent, commonly known as a bleaching agent.
Fixing is typically carried out using a fixing composition that includes one or more fixing agents such as thiosulfate salts. Both ammonium and sodium thiosulfate salts are known. Fixing solutions containing ammonium ions are preferred for providing more rapid fixing, but they present environmental concerns. Thus, fixing solutions containing sodium ions, while slower, are also advantageous.
Color photographic silver halide materials often contain various sensitizing dyes that extend the inherent photosensitivity of the photosensitive silver halide emulsions to electromagnetic radiation. One important class of such sensitizing dyes are carbocyanine sensitizing dyes that are commonly included in silver halide emulsion layers in photographic silver halide films, for example in color reversal photographic silver halide films (films normally used to provide positive color images).
Many photographic silver halide elements contain residual sensitizing dyes after photoprocessing. In some cases, the level of retained sensitizing dyes is inconsequential and thus, unobservable. In other instances, however, the high level of retained sensitizing dye results in undesirably high dye stain (or unwanted color) in the elements.
A number of solutions have been proposed for this problem, including the inclusion of water-soluble stilbene optical brighteners in the color developer solution [as described for example, in Research Disclosure, 20733, page 268, July, 1981 and U.S. Pat. No. 4,587,195 (Ishikawa et al)], the bleach-fixing solution [as described for example, in JP 1-062642 (published Mar. 9, 1989), JP 1-158443 (published Jun. 21, 1989), and U.S. Pat. No. 5,043,253 (Ishikawa)], or the stabilizing solution used at the end of the photoprocessing [as described for example in U.S. Pat. No. 4,895,786 (Kurematsu et al)].
In addition, it has been proposed to include stilbenes in sodium ion containing fixing solutions to solve the problem with retained sensitizing dye, as described in Research Disclosure 37336, page 340, May 1995. Such fixing solutions have sodium ions as the predominant cation because of the environmental concerns with the presence of ammonium ions. However, the presence of sodium ions slows down the fixing process, and this reduction in photoprocessing speed may be unacceptable in some instances. A reduction or elimination of the sodium ions for that reason may be required when certain films (such as color reversal films) are being processed.
We observed, however, that when the noted stilbene compounds were added to conventional ammonium ion containing fixing solutions at appropriate concentrations needed to reduce dye stain, the stilbene compounds were not stable over a desired shelf life. The stilbene compounds stayed in solution for a brief time after mixing, but upon storage for only a few hours, the solutions exhibited considerable precipitation. In fact, the Research Disclosure publication 37336 (noted above) also suggests that stilbene compounds are incompatible in fixing solutions containing high ammonium ion concentration. Thus, it would appear that there is no incentive for a skilled worker in the photographic industry to use stilbenes in fixing solutions containing high ammonium ion content.
There remains a need in the photographic industry for a way to decrease the dye stains resulting from retained sensitizing dye during photoprocessing carried out using high ammonium ion fixing compositions. Such fixing solutions are more desirable for faster photoprocessing of color reversal films. Moreover, while the prior art would suggest that certain stilbene compounds may be useful for reducing the effects of retained sensitizing dye, there is a need in the industry for a means to use them without the instability observed in high ammonium ion fixing compositions.